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24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

How do we perceive Early Reflexions ?
Some Notes on the Directivity of Music Instruments
(Rezeption früher Reflexionen – ein paar Notizen zur
Richtcharakteristik von Musikinstrumenten)
Urban Schlemmer
University of Music and performing Arts, Vienna
schlemmer@nusurf.at

Abstract
Psychoacoustic research of early reflexions (ER) and their spectral attributes
leads to the question how directivity of sound sources contributes to spatial impression. This paper investigates the relationship of the limit of the precedence
effect to echo-perception (LPE) to source-characteristics (as given by the musical
score and the directivity of playing instruments) in the presence of six or more
reflexions reproduced in the listening room by five or more loudspeakers.

1. Introduction
The precedence effect describes the observation that if two or more delayed signals are presented to the hearing system an assessment process takes place, inhibiting redundant information already contained in the preceeding signal [1] - [3]. The inhibited information includes angle of occurrence, delay times and spectral contents of the delayed sound event. A
hearing event evolves with the sensation of space surrounding it.
This sensation is referred to as 'auditive Räumlichkeit' in German by Blauert and many other
authors1. In English and Japanese literature there are several labels for this sensation: the
term auditory spacial impression (ASI) is the umbrella term referring to auditory perception
of the precedence effect. It is subdevided into at least two components: auditory source
width (ASW) and listener envelopment (LEV)2. Bradley [5] introduced the term LEV to
evaluate concert hall acoustics which was later adapted to laboratory experiments by Morimoto [6] and others.
A list of German literature dealing with 'Räumlichkeit' is given in [2], page 75.
A list of English literure dealing with ASI can be found in [4]. Please note that ASI does not translate directly to
'Räumlichkeit' – it is better described as looking at the spacial sensations evolving from the perception of the precedence
effect from another point of view.
1
2

1

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

It is assumed that the precedence effect is closely related to cognition1. Pattern recognition
enables our brains to group similar sensational events together and to perceive them as an
extended attribute of one source. At an even higher level of cognition, patterns are grouped
into streams. Many different perceptual streams coexist at the same time resulting from the
various elicitation of our senses. In a cocktailparty situation e.g. one perceptual stream may
be assigned to background noises and one stream may be assigned to the actual conversation.
Other streams may also be assigned to smelling, tasting, etc. Switching attention to one of
those streams is triggered either intentionally or by events that do not fit into the current pattern or that contain new information about the environment.

2. Hypothesis on the Relation of ESIS and LSIS through ER
It is assumed that two auditory perceptional streams evolve from total spatial impression and
reverberation: First, an early spatial impression stream (ESIS) may be assigned to information concerning the sound source. This is loudness, directivity, minimum and maximum elevation and azimuth. Second, a late spatial impression stream (LSIS) may be assigned to the
perception of room attributes: geometry2 and absorption of walls.
These two perceptional streams are related through the listener's remembered perceptual experience with absobtion and room geometry on one hand and remembered subjective ASI
of the sound sources on the other hand. It is therefore assumed that level and spectral attributes of ER contribute to the interconnexion of ESIS and LSIS through knowledge [7].
Level and spectral attributes of ER relate to directivity of music instruments. This interconnexion may be an important factor for a natural sounding room simulation with five or more
loudspeakers.

1
2

See also [3], page 49ff
Geometry includes volume, length, highth and width as well as balconies, columns and sound-scattering surfaces.

2

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

Transition Period of ESIS to LSIS
In a transition period ER will contribute to both ESIS and LSIS. The transition time may be
defined as the crossing point of the LPE with the level of the physical sound field as shown
in the following sketch. This may be the time when the diffuse sound field becomes just audible as a seperate auditory event. Blauert mentions this relationship as a “characteristic
blurr of auditory events in the timedomain caused by late reflexions and reverberation” 1.
Lehmann [8] stated that a transition time when it makes no longer sense to analyse descrete
reflexions in a sound field - which will then be discribed by statistical terms - is approximately given by t gr =2  V .
+15

Diffuse Field

0dB

-15

LPE for strings
ER (n
on-dir
ection
al rad
iation
)
Duration of 8 th Note at 120 bpm

-30

0

50

100

150

200

250

300

350

400

450

500
ms

Figure 1 shows calculated ER for the Vienna Musikverein Hall at a Distance of 10m from the sound source, which is
approximately the hall distance for the directed radiation of strings. At the hall distance the diffuse field has the same
level as the direct sound. An estimate of the LPE is shown by the blue line. The directivity of violins and violas contains level differences of up to 10 dB in the 2kHz and 4kHz band for the direction of direct sound compared to the reflexion of the ceiling, contributing to LEV in this room [11].

In German: “charakteristisches Verschleifen der Hörereignisse infolge von späten Rückwürfen und Nachhall” [2],
page.75
1

3

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

3. LPE in Relation of sound source Characteristics
Morimoto [6] showed in 2003 that reflexions at LPE contribute to a subjective feeling of
LEV. A high level of early reflexions in diffuse sound fields is generally preferred in subjective listening tests [9]1. However, the perception of echos must be avoided. Therefore controlling ER at LPE is a key in reproduced ASI.
3.1 Review of Investigations on LPE
Fig. 3 shows the results of different investigations on LPE in anechoic environments. When
the level of the delayed signal approaches LPE, at first, a sensation of image split occurs.
Then a second auditory event becomes audible. Finally, the echo is described as 'disturbing'.
Limit of the precedence effect to 'image split' or 'echo perception' with one single delayed signal
Schubert 1969 100ms rauschen HP

+40

100ms rauschen LP
Cherry 1954 speech

Morimoto 2003
music motif 135 Grad
(mozart, jupiter symphony)

100ms Ton 250 Hz

+20

50ms Ton 250 Hz

up to

0 dB
-20

Lochner 1958 speech 50dB

Babkoff 1966
clicks

speech 25dB
Meyer 1952
speech

Damaske 1971noise 100ms
noise 10ms

-40

2 ms

20

40

60

80

100 ms

Figure 2: If not noted otherwise the direct sound is frontal and the angle of the reflexion is set to 40 degrees

From the preceeding studies, it can be seen that LPE varies greatly with source charcteristics.
An incomplete list of some relations may be described as followes:

1

A survey of measurements of subjective spatial attributes can be found in [10].

4

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

Condition

Investigation

The greater

the number of reflexions the more unlikely echos are audible.

The greater
The greater

the spectral incoherence the more likely
the transients
the more likely

The greater

the time gap between direct and reflexion
the level of ER
the overall loudness
the note durations
the frequency contents of
ER
the angle of occurence
(0...180 degrees)

The higher
The higher
The shorter
The higher
The greater

the more likely

Ebata, Sone, Nimura,
1968 [1,p.219]
echos are audible. See 3.2.1
echos are audible. Schubert, Wernick, 1969
[1,p. 185]
echos are audible. [All investigations]

the more likely
the more likely
the more likely
the more likely

echos are audible.
echos are audible.
echos are audible.
echos are audible.

the more likely

[All investigations]
See 3.2.2
Damaske 1971[1,p. 182]
Schubert and Wernick,
1969[1,p. 185]
echos are audible. Boerger 1965 [1,p. 183]

Table1: List of qualitative relations of LPE to different source charcteristics

3.2 Experiments with anechoic Drum Sounds
Because it can not be predicted from the preceeding investigations which values of the LPE
can be expected if music instruments in a studio are used for simulated reflexions, LPE was
measured with three anechoically recorded instruments in a preliminary study. Additionally
a -3dB highshelf filter was applied to the delayed signal.
Bass Drum, Hihat and Snare at 50 dB SPL

3.2.1

Fig. 3 shows the results of the first measurement.
-2,5

Bass Drum 50dB
Snare Drum 50dB
-------------Hihat 50dB

-5
-7,5
-10
-12,5
-15
-17,5

Lp in [dB]

-20
-22,5
-25
-27,5
-30
-32,5
-35
-37,5
-40
-42,5
-45
-47,5
-50
10

20

30

40

50

60

70

80

90

100

110

120

130

time in [ms]

5

140

150

160

170

180

190

200

210

220

230

240

250

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

Bass Drum and Snare Drum at 35 dB SPL

3.2.2

Fig. 4 shows the results of the second measurement.
5
2,5

BD 061008 35dB
SD 061009 35dB
---------------------------

0
-2,5
-5
-7,5
-10
-12,5
-15
-17,5
-20
-22,5
-25
-27,5
-30
-32,5
-35
-37,5
-40
-42,5
10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

Figure 4: LPE measured with a single bassdrum-hit and a snaredrum hit at 35 dB SPL.

3.2.3

Discussion

The measurements correspond qualitatively to the findings in Table 1. LPE decreases with
total loudness for high frequency components, meaning that ASI increases with total loudness. This corresponds to findings by Marschall, Keet and Barron as described in [2]. Therefore the contradicting findings as depicted by Blauert in [1] referring to investigations by
Lochner and Burger from 1958 may be wrong.
In this study, LPE for low frequency components stays mostly constant for different total
loudness exept for short delaytimes smaller than 30ms. LPE for the snare drums is about 15
dB under LPE for the bass drum at a total loudness of 50 dB SPL. This difference results
from a wider spectrum, a shorter note duration and more high frequency components compared to the bass drum. The Hihat has a narrower spectrum than the snaredrum but contains
more high frequency components than the bass drum and therefore lies between the two other measurements. LPE for comparable frontal reflexions as shown in Fig. 2 is about 10 dB
higher than the measured values for 110 degrees.
3.3 Summary on Source Characteristics
The precedence effect is continously retriggered by new sound events in a concert. This
means that reflexions arriving at the onset of notes are closer to LPE than in the sustain
phase. Therefore long sustained notes do not contribute to ASI as much as a series of short
notes.

6

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

Directivity affects LPE in the following ways:


The spectral contents of the reflexion differs from the direct sound.



The overal loudness of the reflexion differs from the direct sound.

This means if ER result from the directivity that lower LPE (e.g. flute, strings, horn), ASI of
these music instruments is perceived as high. If, on the other hand, ER result from the directivity that highten LPE (e.g. trombone, trumpet), ASI of these music instruments is perceived
as low.

4. Experiments with Reflexions of Higher Order
4.1

Enlargement of the Listening Area through ER

4.1.1

Experimental Setup

For this test, a recording of flute solo1 (J.B. Bach, BWV 1013) was used, a mono mix from
two microfones with a reverberation without ER. The directivity of the flute was considered
and transcribed by a filterbank for each of the reflexions, calculated by the urban-reflexionsprogram, as described in [12]. The pattern was animated through the simulation of a constantly moving sound source.
The following ER pattern was applied:
1. Order
1. Order

1. Order
1. Order

1. Order

1. Order

dly time
1.247 ms
14.76 ms
20.32 ms
26.32 ms
32.83 ms
39.38 ms
48.98 ms
49.07 ms
49.76 ms
52.88 ms
57.24 ms
57.89 ms
59.02 ms
61.50 ms
97.43 ms
97.91 ms
103.6 ms
106.3 ms
106.9 ms
109.3 ms
114.8 ms

2. Order
2. Order
3. Order
2. Order
2. Order
3. Order
3. Order
2. Order
3. Order
3. Order
2. Order
2. Order
3. Order
2. Order
3. Order

Table2: calculated delaytimes and increased
density through reflexions of 2nd and 3rd order
1

Studios der MDW, Birgit Herrmannseder, Flöte, Tonmeister: U. Schlemmer, 2003

7

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

The angle of appearance was resolved considering the speaker setup shown in Fig.9. In addition, a standart five-speaker-ITU setup was employed.

Fig. 9: eight speaker setup used for
experiment 4.1
4.1.2

First impressions



Although the latest simulated reflexions had delay times of 160 ms and distances between
reflexions appeared of 25 ms, no echos could be heard.



The effect is subtle and does not cause unwanted coloration. It is hardly consciously perceived.



Single loudspeakers are not localized, even if one moves far outside the sweetspot.



Less reverberation is needed in the mix.



Eight channels sound much better than five. Particularly speakers at the sides seem to be
important for the representation of lateral reflexions.



The feeling of envelopment seems realistic and location-independent.



The surround-centre seems to be important as you move around and turn around in the
soundfield.



If you turn around, the feeling of beeing 'misplaced' disappears. One enjoys an enlarged
listening area - no need to stay in the sweetspot. You can walk around.



It works best for one solo-instrument. Stereo-techniques can be tricky to integrate.

Most importantly these reflexions or delayed signals are not being allowed to have the same
frequency response. If none or the same filtering is used for all delays, then:


the image of the instrument is blurred



ASI sounds artificial



when moving around combfilter-effects are audible.

8

24th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November, 2006

4.1.3

Discussion

As Franssen showed in 1960 [13] after a successful localization, the recognized angle of incidence is assumed as long as a new evaluable localization-stimulus is given. This may be
an explanation for the fact that localization remains stable even if one turns around 180 degrees. This requires no disturbing reflexions to occur when moving one's head. This would is
the case when the ER-pattern can be assigned to a real room or if the ER-pattern has the
same characteristics as in a real room, because in this case the occurrance of reflexions is
predictable. [14] - [16] When a new localizition stimulus occurs the acoustical situation in
the new position is approved even if a localization is not possible due to the head being in an
infavourable position. The head movement therefore acts as a validation of the reproduced
ER-pattern: when the reflexions or delayed signals can be comprehensibly assigned to a natural room or a remembered environment1, the previously occurring ASI may be affirmed.
Another experience is, that with a non-animated ER-pattern one can omit the ER-pattern after a while and nothing seems to be missing. This is also because the pattern is being remembered even when no new stimulus is given. Only when the listener moves around, a validation of the remembered ER pattern takes place. This corresponds to the above, as when you
move, the previously stored pattern is compared to the actual situation for orientation purposes [2][14].
Following Mackensen, spontaneous head movements improve localization. [7] With two
speakers, the evaluation of the ER-pattern through head movements may not be possible and
the validation will fail. In contrast, it feels pleasent if the localization stays stable in spite of
head movements and walking around.

4.2 Recognition of Source Directivity
4.2.1

Experiment I

The ER-pattern of Table 2 was calculated with two different directivity patterns, violin and
piano. It was then applied to an anechoic recording of the 'pizzicato polka' by R.Strauß,
played solely with pizzicato strings2.
4.2.2

Experiment II

The ER-pattern of Table 2 was calculated with two different directivity patterns, flute and
trombone. It was then applyed to the flute recording described in section 4.1.1.

1
2

Mackensen mentions 'knowledge, memory and learning effects' as additional cues in localization [7].
Taken from the Dennon CD.

9


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